Film for pellicle

ABSTRACT

A pellicle film comprised of a resin prepared by reducing a polyvinyl acetal of the formula: ##STR1## wherein R is a C1-3 alkyl group and the vinyl acetate content &#34;y/(x+y+z)&#34; is not larger than 0.1 and the degree of acetalization &#34;z/(x+y+z)&#34; is at least 0.6. This pellicle film exhibits a reduced UV absorption in the vicinity of 365 nm and is durable against irradiation with UV rays, especially i-line of a mercury arc light.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a film for a pellicle which is used as a dustcover for a photomask or reticle (which is hereinafter referred to as"mask" for brevity) used at the lithographic step in the course for theproduction of a semiconductor integrated circuit.

The film for a pellicle of the present invention can also be used inlithography utilizing as a light source i-line having a wavelength of365 nm of a mercury arc light, which is recently used.

(2) Description of the Related Art

In the production of a semiconductor integrated circuit, the step ofpatterning by light exposure a semiconductor wafer coated with a resistmaterial is important because it dominates the yield of the integratedcircuit. If scratches or dusts are present on a mask which is anoriginal sheet for patterning, the images of the scratches or dusts aswell as those of the intended pattern are transferred onto the wafer andconsequently shortcircuiting or wiring breakage occasionally occurs inthe integrated circuit. Therefore, protection of the mask is importantfor enhancing the yield of the integrated circuit and a pellicle is nowoften used as a dust cover for protection of the mask.

As pellicle film-forming materials, nitrocellulose, cellulose acetate,cellulose propionate, polyethylene terephthalate, polypropylene andpolymethyl methacrylate have heretofore been proposed. Of these,nitrocellulose is used as a pellicle film in a light exposure apparatusutilizing g-line (wavelength: 436 nm) of a mercury arc light as thelight source.

Recently, the demand for high densification of a semiconductorintegrated circuit requires shortening of the wavelength in lithographylight source, and thus, the i-line having a wavelength shorter than thatof the g-line is used as the lithography light source, as well as theconventional g-line.

Conventional pellicle film-forming materials such as nitrocellulose canbe used in the lithography using as the light source near ultravioletrays such as g-line and h-line of a mercury arc light, but cannotadvantageously used in a light exposure apparatus using i-line of amercury arc light, as the lithography light source. This is because theconventional pellicle film-forming materials exhibit a light absorptionin the deep ultraviolet region, and therefore, rapidly become opaque orare deteriorated when exposed to the i-line. Therefore, a pellicle filmis now eagerly desired which can advantageously be used for i-line aswell as g-line or h-line of a mercury arc light.

The following are usually required for a film for a pellicle: (1) thepellicle film does not exhibit a light absorption, i.e., is transparent,in the wavelength region of the light source of the light exposureapparatus, and (2) the pellicle film is durable, that is, even when thepellicle is continuously exposed to light for a long time, it does notbecome opaque nor is deteriorated. When the pellicle film is used in alight exposure apparatus using i-line of a mercury arc light as thelight source, it is required that the pellicle film is transparent toand durable against the i-line.

As a film for a pellicle used in a light exposure apparatus utilizingthe i-line as the light source, a pellicle film having a multilayerstructure has been proposed which is comprised of a nitrocellulose filmhaving reflection-preventing layers composed of a fluorine-containingpolymer on both surfaces thereof (for example, see Japanese UnexaminedPatent Publication No. H1-100549). The process for preparing thispellicle film is complicated, the yield is low and the production iscostly.

As another pellicle film used in a light exposure apparatus utilizingi-line of a mercury arc light, a pellicle film comprised of a polyvinylacetal resin has been proposed (for example, see Japanese UnexaminedPatent Publication No. H1-172430). The polyvinyl acetal resin pelliclefilm can be used without reflection-preventing layers and the productionthereof is neither complicated nor costly, but this pellicle film is notdurable, that is, when exposed to the i-line for a long period, itbecomes opaque and is deteriorated.

SUMMARY OF THE INVENTION

Under the above-mentioned background, a primary object of the presentinvention is to provide a film for a pellicle which can be used withoutreflection-preventing layers in a light exposure apparatus utilizingi-line of a mercury arc light as the light source and is durable, thatis, does not become opaque nor is deteriorated even when exposed to thei-line for a long time.

In accordance with the present invention, there is provided a film for apellicle consisting essentially of a resin prepared by reducing apolyvinyl acetal resin represented by the following formula (1):##STR2## wherein R is a linear or branched alkyl group having 1 to 3carbon atoms, and x, y and z are numbers such that the content of thevinyl acetate units, expressed by y/(x+y+z), is not larger than 0.1 andthe degree of acetalization, expressed by z/(x+y+z), is at least 0.6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows ultraviolet transmissions of reduced polyvinyl propionalresins prepared in Synthesis Examples 2, 3 and 4, and that of anuntreated polyvinyl propional resin prepared in Synthesis Example 1;

FIG. 2 shows an ultraviolet transmission of a reduced polyvinylpropional prepared in Synthesis Example 5 and that of an untreatedpolyvinyl propional resin prepared in Synthesis Example 1;

FIG. 3 shows an ultraviolet transmission of a reduced polyvinyl butyralresin prepared in Synthesis Example 7 and that of an untreated polyvinylbutyral resin prepared in Synthesis Example 6;

FIG. 4 shows ultraviolet transmissions of reduced polyvinyl propionalresins prepared in Synthesis Examples 8 and 9, and that of an untreatedpolyvinyl propional resin prepared in Synthesis Example 1; and

FIG. 5 shows an ultraviolet transmission of a reduced polyvinylpropional resin prepared in Synthesis Example 10 and that of anuntreated polyvinyl propional resin prepared in Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The reduced polyvinyl acetal resin used in the present invention isprepared from a polyvinyl acetal resin represented by the followingformula (1): ##STR3## wherein R is a linear or branched alkyl grouphaving 1 to 3 carbon atoms such as methyl, ethyl, n-propyl and isopropylgroup, and x, y and z are numbers such that the content of the vinylacetate units, expressed by y/(x+y+z), is not larger than 1.0 and thedegree of acetalization, expressed by z/(x+y+z), is at least 0.6.

The polyvinyl acetal resin of the formula (1) is prepared by a knowncondensation reaction between polyvinyl alcohol and an aldehyde such asacetaldehyde, propionaldehyde, n-butyl aldehyde or sec-butyl aldehyde.For example, an aldehyde and an acid catalyst such as sulfuric acid orhydrochloric acid are simultaneously added into a solution of polyvinylacetate in a solvent such as methanol, ethanol or acetic acid wherebysaponification and acetalization of the polyvinyl acetate areconcurrently effected. Alternatively, polyvinyl alcohol is dissolved ordispersed in water or an alcohol such as methanol, ethanol, n-propylalcohol or iso-propyl alcohol, or a water/alcohol mixture; theabove-mentioned aldehyde and acid catalyst are added to the solution ordispersion of polyvinyl alcohol whereby the condensation of polyvinylalcohol with an aldehyde is effected. The latter method employingpolyvinyl alcohol as a starting material is preferable because theresulting resin exhibits a good transparency and it is easy to controlthe molecular weight of the resin and also control the content of thevinyl acetate units in the resin. The control of the content of thevinyl acetate units in the resulting polyvinyl acetal resin is effectedby varying the content of the vinyl acetate units in the startingpolyvinyl alcohol. By varying the charge ratio of polyvinyl alcohol toan aldehyde, the reaction time, the reaction temperature and the amountof the catalyst, the degree of acetalization of the polyvinyl acetalresin can be controlled.

The content of the vinyl acetate units in the polyvinyl acetal resin,expressed by y/(x+y+z), is not larger than 0.1, preferably not largerthan 0.01 and more preferably not larger than 0.005. The degree ofacetalization of the polyvinyl acetal resin, expressed by z/(x+y+z), isat least 0.6. If the content of the vinyl acetate units and the degreeof acetalization do not satisfy these requirements, the pellicle becomesdifficult to release from a substrate at the pellicle film-forming stepand irreversible elongation or fogging occur at the step of washing thepellicle film with water for removing dust.

The weight average molecular weight of the polyvinyl acetal resin, asdetermined by the gel permeation chromatography using monodispersepolystyrene as the reference material, is preferably in the range from10,000 to 300,000. Where the weight average molecular weight is outsidethis range, the pellicle film has a poor uniformity and mechanicalstrength.

The reduced polyvinyl acetal resin used in the present invention isprepared by reducing the polyvinyl acetal resin of the formula (1). Asthe reduction method, there can be mentioned (a) a method using a metalhydride complex, (b) a catalytic hydrogenating reduction method using ametal catalyst, and (c) a method using a solution of an alkali metal inan amine.

As the metal hydride complex used as the reducing agent in the method(a), there can be mentioned lithium aluminum hydride, sodium aluminumhydride, sodium borohydride, lithium borohydride, NaBH₄ --AlCl₃, NaBH₄--BF₃, LiAl(O--t--Bu)₃, NaAlH₂ (OCH₂ CH₂ OCH₃)₂, NaBH(OCH₃)₃ andLiBH(sec-Bu)₃. These reducing agents can be used alone or incombination. Of these reducing agents, lithium aluminum hydride ispreferable because this compound has a strong reducing power and isinexpensive and readily commercially available.

The method (a) of reducing the polyvinyl acetal of the formula (1) usingthe metal hydride complex is specifically described in the followingwith reference to the use of lithium aluminum hydride.

The polyvinyl acetal resin of the formula (1) is reduced in the form ofa solution in an ether solvent or in the state of being swollen with anether solvent. The ether solvent includes, for example, tetrahydrofuran(THF), diethyl ether, di-n-propyl ether, di-n-butyl ether and1,4-dioxane. The lithium aluminum hydride catalyst is dispersed in theether solvent and the catalyst dispersion is incorporated with thedissolved or swollen polyvinyl acetal thereby to effect reduction. Theamount of the lithium aluminum hydride is in excess of the equivalentamount necessary for reducing the carbonyl groups in the polyvinylacetal resin. The ether solvent must be preliminarily dehydrated andpurified. The reduction can be carried out at a temperature of from roomtemperature to the reflux temperature. The reaction time is at least onehour, although the suitable reaction time varies depending upon thereaction temperature.

After completion of the reaction, water is added to the reaction mixtureand thereafter the reduced resin is extracted with an organic solvent.The organic solvent includes, for example, methylene chloride,chloroform, carbon tetrachloride, benzene, toluene and xylene. Theorganic phase is dehydrated with a dehydrating agent such as calciumchloride, magnesium sulfate or sodium sulfate, and then filtrated with aglass filter or filter paper, followed by removal of the dehydratingagent. Further, the filtrate is filtrated with a membrane filter havinga pore size below 1 micron and then distilled under a reduced pressureto remove the solvent and obtain the intended reduced polyvinyl acetalresin. If desired, the resin is purified, for example, byreprecipitation.

The reduction of the polyvinyl acetal resin of formula (1) by thecatalytic hydrogenating reduction method (b) using a metal catalyst canbe carried out by a conventional manner. As the metal catalyst, therecan be mentioned a Ranie-nickel catalyst (R-Ni), a palladium carboncatalyst (Pd/C), a platinum oxide catalyst (PtO₂), a copper-chromiumoxide catalyst (CuO.Cr₂ O₄) and a nickel-diatomaceous earth catalyst.

The reducing method (c) using a solution of an alkali metal in an amineis specifically described in the following. As the alkali metal, therecan be mentioned sodium and lithium. As the amine, there can bementioned liquid ammonia, methylamine, ethylamine, dimethylamine,ethylenediamine, propylamine, morpholine and hexamethylphosphorictriamide. Among the alkali metal/amine combinations, a combination ofsodium with hexamethylphosphoric triamide is preferable because thereduction treatment is easy and this combination exhibits a strongreducing power.

The manner in which the alkali metal/amine combination is used is notparticularly limited, but may be conventional. For example, the methoddescribed in G. M. Whitesides and W. J. Ehmann, J. Org. Chem., 35, 3565(1970) can be employed. In a typical treating manner, a solution of thepolyvinyl acetal resin of the formula (1) is added dropwise slowly intoa solution of metallic sodium in hexamethylphosphoric triamide at roomtemperature, or metallic sodium is gradually added into a solution ofthe polyvinyl acetal resin of the formula (1).

The pellicle film of the present invention can be made by a processwherein a solution of the reduced polyvinyl acetal resin in an organicsolvent such as cyclohexanone, ethylene glycol monoethyl ether orethylene glycol monoethyl ether acetate is prepared, and then the resinsolution is coated by using a spin coater on a flat substrate such assilicon wafer to form a uniform film having a thickness of about 1.5microns, followed by release of the film from the substrate.

The pellicle film of the present invention is characterized asexhibiting a reduced ultraviolet absorption in the vicinity of 365 nmand being durable against irradiation with ultraviolet rays, especiallyi-line of a mercury arc light, as compared with a pellicle filmcomprised of an unreduced polyvinyl acetal resin. Therefore, thepellicle film of the invention can be employed in a light exposureapparatus utilizing the i-line as the light source, as well as anapparatus utilizing g-line or h-line of a mercury arc light.

The present invention will now be described in detail with reference tothe following examples that by no means limit the scope of theinvention.

In the examples, the molar fractions of the vinyl alcohol units andvinyl acetate units in the polyvinyl acetal resin were determinedaccording to JIS (Japanese Industrial Standard) K-6728, and the degreeor acetalization was determined by subtracting these molar fractionsfrom the entirety.

SYNTHESIS EXAMPLE 1 Synthesis of Polyvinyl Propional

In 500 ml of a methanol/water (95/5 by weight) mixture was dispersed 50g of polyvinyl alcohol (completely saponified, average degree ofpolymerization: 1,000, "PVA-110" supplied by Kuraray). To thisdispersion were added 10 ml of an aqueous 35% hydrochloric acid solutionand 115.35 g of propionaldehyde, and the resulting mixture wasmaintained at 55° C. for 96 hours with stirring to effect acetalization.After completion of acetalization, sodium acetate was added to thereaction mixture thereby to be neutralized, the reaction medium wasremoved by decantation and the precipitated polymer was recovered. Thepolymer was dissolved in 1.5 liters of acetone and the resulting polymersolution was added dropwise into 8 liters of water to precipitate thepolymer. The polymer was vacuum-dried at 40° C. for 48 hours to yield 55g of polyvinyl propional (PVP-1). The content of vinyl acetate units inPVP-1 was 0.003 and the degree of acetalization of PVP-1 was 0.90 . Theweight average molecular weight (Mw) of PVP-1 was 145,000 as determinedby gel permeation chromatography, and the molecular weight distribution(Mw/Mn) was 2.4.

To evaluate the transparency of PVP-1, a 3% by weight solution of PVP-1in methylene chloride was prepared and the light absorption and thelight transmission were determined by an ultraviolet-visible lightspectrophotometer ("UV-260" supplied by Shimadzu Corp.) provided with aquartz cell having an optical path of 1 cm wherein methylene chloridewas used as a reference material. The light absorption at 365 nm was0.05 and the light transmission was 87.8%. The spectral transmissioncurve of PVP-1 is shown as curve (a) in FIG. 1.

SYNTHESIS EXAMPLE 2 Reduction (1) of Polyvinyl Propional by UsingLithium Aluminum Hydride

In 150 ml of dehydrated and purified tetrahydrofuran (THF) was dissolved10.0 g of PVP-1 prepared in Synthesis Example 1, and the PVP-1 solutionwas cooled below 10° C. Separately, 1.0 g of a lithium aluminum hydride(LiAlH₄) powder was dispersed in 150 ml of THF, and the LiAlH₄dispersion was added dropwise into the PVP-1 solution at a temperaturebelow 10° C. The resultant mixture was refluxed for 2 hours to effectreduction. To the solution, 50 ml of methanol was added, the mixture wasstirred overnight and the reaction medium was removed by vacuumdistillation. To the thus-obtained solid, 200 ml of water was added andthe mixture was extracted three times with 100 ml of methylene chloride,and the methylene chloride phase was dried with calcium chloride andfiltrated by a glass filter and then by a membrane filter having a poresize of one micron. The solvent was removed from the filtrate by vacuumdistillation to yield 8.2 g of the intended reduced polyvinyl propional(RVP-1).

The content of vinyl acetate units in RVP-1 was 0.001 and the degree ofacetalization of RVP-1 was 0.90. The transparency of RVP-1 was evaluatedin the same manner as described in Synthesis Example 1 and it was foundthat the light absorption was 0.02 and the light transmission was 95.6%.The spectral transmission curve of RVP-1 is shown as curve (b) in FIG.1.

SYNTHESIS EXAMPLE 3 Reduction (2) of Polyvinyl Propional by UsingLithium Aluminum Hydride

Reduction of PVP-1 was carried out in the same manner as described inSynthesis Example 2 wherein, after the addition of the dispersion ofLiAlH₄ in THF to the PVP-1 solution, the reaction was carried out atroom temperature for 72 hours with all other conditions remainingsubstantially the same. Thus, 8.0 g of a reduced polyvinyl propionalresin (RVP-2) was obtained. The content of vinyl acetate units in RVP-2was 0.001 and the degree of acetalization of RVP-2 was 0.90. Thetransparency of RVP-2 was evaluated in the same manner as described inSynthesis Example 1 and it was found that the light absorption was 0.02and the light transmission was 95.2%. The spectral transmission curve ofRVP-2 is shown as curve (c) in FIG. 1.

SYNTHESIS EXAMPLE 4 Reduction (3) of Polyvinyl Propional by UsingLithium Aluminum Hydride

Reduction of PVP-1 was carried out in the same manner as described inSynthesis Example 2 wherein di-n-propyl ether was used instead of THFand the reducing reaction was carried out for 72 hours under reflux withall other conditions remaining substantially the same. Thus, 8.3 g of areduced polyvinyl propional resin (RVP-3) was obtained. The content ofvinyl acetate units in RVP-3 was 0.001 and the degree of acetalizationof RVP-3 was 0.90. The transparency of RVP-3 was evaluated in the samemanner as described in Synthesis Example 1 and it was found that thelight absorpiton was 0.02 and the light transmission was 95.3%. Thespectral transmission curve of RVP-3 is shown as curve (d) in FIG. 1.

SYNTHESIS EXAMPLE 5 Reduction of Polyvinyl Propional by Using SodiumAluminum Hydroxide

Reduction of PVP-1 was carried out in the same manner as described inSynthesis Example 2 wherein 1.4 g of sodium aluminum hydride (NaAlH₄)was used instead of 1.0 g of lithium aluminum hydride with all otherconditions remaining substantially the same. Thus, 8.3 g of a reducedpolyvinyl propional (RVP-4). The content of vinyl acetate units in RVP-4was 0.001 and the degree of acetalization of RVP-4 was 0.90. Thetransparency of RVP-4 was evaluated in the same manner as described inSynthesis Example 1 and it was found that the light absorption was 0.02and the light transmission was 94.4%. The spectral transmission curve ofRVP-4 is shown as curve (e) together with curve (a) of PVP-1 in FIG. 2.

SYNTHESIS EXAMPLE 6 Synthesis of Polyvinyl Butyral

In 500 ml of methanol/water (95/5 by weight) mixture was dispersed 50 gof polyvinyl alcohol (completely saponified, average degree ofpolymerization: 1,000, "PVA-110" supplied by Kuraray). To thisdispersion were added 10 ml Of an aqueous 35% hydrochloric acid solutionand 140.25 g of butyl aldehyde, and the resulting mixture was maintainedat 55° C. for 96 hours with stirring to effect acetalization. Aftercompletion of acetalization, sodium acetate was added to the reactionmixture thereby to be neutralized, the reaction medium was removed bydecantation and the precipitated polymer was recovered. The polymer wasdissolved in 1.5 liters of acetone and the resulting polymer solutionwas added dropwise into 8 liters of water to precipitate the polymer.The polymer was vacuum-dried at 40° C. for 48 hours to yield 54 g ofpolyvinyl butyral (PVB-1). The content of vinyl acetate units in PVB-1was 0.009 and the degree of acetalization of PVB-1 was 0.79 . The weightaverage molecular weight (Mw) of PVB-1 was 162,000 as determined by gelpermiation chlomatography, and the molecular weight distribution (Mw/Mn)was 2.6.

To evaluate the transparency of PVB-1, a 3% by weight solution of PVB-1in methylene chloride was prepared and the light absorption and thelight transmission were determined by an ultraviolet-visible lightspectrophotometer ("UV-260" supplied by Shimadzu Corp.) with a quartzcell having an optical path of 1 cm wherein methylene chloride was usedas a reference material. The light absorption at 365 nm was 0.08 and thelight transmission was 82.6%. The spectral transmission curve of PVB-1is shown as curve (f) in FIG. 3.

SYNTHESIS EXAMPLE 7 Reduction of Polyvinyl Butyral by Using LithiumAluminum Hydride

Reduction of 10 g of PVB-1 prepared in Synthesis Example 6 was carriedout by using LiAlH₄ in the same manner as described in Synthesis Example2 to yield 7.8 g of a reduced polyvinyl butyral resin (RVB-1). Thecontent of vinyl acetate units in RVB-1 was 0.002 and the degree ofacetalization of RVB-1 was 0.79. The transparency of RVB-1 was evaluatedin the same manner as described in Synthesis Example 1 and it was foundthat the light absorption was 0.02 and the light transmission was 95.6%.The spectral transmission curve of RVB-1 is shown as curve (g) togetherwith curve (f) of PVB-1 in FIG. 3.

SYNTHESIS EXAMPLE 8 Catalytic Hydrogenating Reduction of PolyvinylPropional by Using Ranie-Nickel Catalyst

In 1,000 ml of thoroughly dried 1,4-dioxane was dissolved 80 g of PVP-1prepared in Synthesis Example 1, and the entire mixture was charged in atwo liter stainless steel autoclave provided with a stirrer. Separately,a Ranie-nickel W-2 catalyst was newly developed and the solvent thereofwas substituted by 1,4-dioxane, and 16 g of the thus-treated catalystwas added into the PVP-1 charged autoclave. After the autoclave wasclosed, a high-purity hydrogen gas was introduced into the autoclave andthe content was then maintained at 90° C. under a hydrogen pressure of60 kg/cm² for 8 hours with stirring. It was found that about 0.3 literof hydrogen gas was consumed and thereafter the consumption of hydrogengas became saturated. The thus-prepared reduction product was restoredto the normal state (i.e., room temperature and normal pressure) andfiltrated by using a Celite filter medium, and the filtrate wasfiltrated by a membrane filter having a pore size of 0.1 micron wherebythe catalyst was completely removed. The thus-obtained filtrate wasadded dropwise into 10 liters of water under vigorous agitation toprecipitate a polymer. The polymer was washed repeatedly with water andthen vacuum-dried at 40° C. for 48 hours to yield 72 g of a reducedpolyvinyl propional (RVP-5). The content of vinyl acetate units in RVP-5was 0.002 and the degree of acetalization of RVP-5 was 0.90.

The transparency of RVP-5 was evaluated in the same manner as describedin Synthesis Example 1 and it was found that the light absorption was0.02 and the light transmission was 96.2%. The spectral transmissioncurve of RVP-5 is shown as curve (h) together with curve (a) of PVP-1 inFIG. 4.

SYNTHESIS EXAMPLE 9 Catalytic Hydrogenating Reduction of PolyvinylPropional by Using Palladium Carbon Catalyst

In 1,250 ml of throughly dried ethylene glycol monoethyl ether wasdissolved 100 g of PVP-1 prepared in Synthesis Example 1, and the entiremixture was charged in a two liter stainless steel autoclave providedwith a stirrer. Into the PVP-1 charged autoclave, was incorporated 20 gof 10% palladium carbon. After the autoclave was closed, a high-purityhydrogen gas was introduced into the autoclave and the content wasmaintained at room temperature under a hydrogen pressure of 80 kg/cm²for 8 hours with stirring. It was found that about 0.5 liter of hydrogengas was consumed and thereafter the consumption of hydrogen gas becamesaturated. The thus-prepared reduction product was filtrated by using aCelite filter medium and the filtrate was filtrated by a membrane filterhaving a pore size of 0.1 micron whereby the catalyst was completelyremoved. The thus-obtained filtrate was added dropwise into 10 liters ofwater under vigorous agitation to precipitate a polymer. The polymer wasrepeatedly washed with water and then vacuum-dried at 40° C. for 48hours to yield 89 g of a reduced polyvinyl propional (RVP-6). Thecontent of vinyl acetate units in RVP-6 was 0.002 and the degree ofacetalization of RVP-6 was 0.90.

The transparency of RVP-6 was evaluated in the same manner as describedin Synthesis Example 1 and it was found that the light absorption was0.02 and the light transmission was 94.8%. The spectral transmissioncurve of RVP-6 is shown as curve (i) together with curve (a) of PVP-1 inFIG. 4.

SYNTHESIS EXAMPLE 10 Reduction of Polyvinyl Propional by Using Solutionof Metallic Sodium in Hexamethylphosphoric Triamide

A two liter four-necked flask provided with a stirrer, a condenser, agas-introducing tube and a dropping funnel was charged with 1,000 ml ofthoroughly dried hexamethylphosphoric triamide, and the content wasstirred while dry nitrogen gas was introduced above the liquid surface.Into the flask, 5 g of metallic sodium was gradually added and theliquid mixture was thoroughly stirred at room temperature until theliquid became deep blue to prepare a solution of metallic sodium inhexamethylphosphoric triamide. Separately, 50 g of PVP-1 prepared inSynthesis Example 1 was dissolved in 500 ml of thoroughly purified anddried dioxane, and the thus-prepared PVP-1 solution was added dropwisethrough the dropping funnel into the metallic sodium solution inhexamethylphosphoric triamide at room temperature over a period of 4hours. After the dropping addition, the mixture was maintained at roomtemperature for 16 hours to continue the reduction reaction, and then100 ml of ethylene glycol monoethyl ether was added to the reactionmixture. The reaction mixture was added dropwise into 4 liters of waterto precipitate a polymer. The polymer was repeatedly washed with waterand then vacuum-dried at 40° C. for 48 hours to yield 45 g of a reducedpolyvinyl propional (RVP-7). The content of vinyl acetate units in RVP-7was 0.001 and the degree of acetalization of RVP7 was 0.90.

To evaluate the transparency of RVP-7, a 3% by weight solution of RVP-7in methylene chloride was prepared and the light absorption and thelight transmission were determined by an ultraviolet-visible lightspectrophotometer in the same manner as described in SynthesisExample 1. The light absorption at 365 nm was 0.01 and the lighttransmission was 98.8%. Thus, the transparency of RVP-7 was highlyenhanced as compared with that of unreduced polyvinyl propional (PVP-1).The spectral transmission curve of RVP-7 is shown as curve (j) togetherwith curve (a) of PVP-1 in FIG. 5.

EXAMPLE 1

In 290 g of cyclohexanone was dissolved 30 g of the reduced polyvinylpropional (RVP-1) prepared in Synthesis Example 2. The RVP-1 solutionwas spin-coated on a silicon wafer having a diameter of 6 inches byusing a spin-coater at a revolution of 3,000/min and then vacuum-driedto form a film having a thickness of 1.5 microns on the silicon wafer.The film was separated from the wafer to give a pellicle film.

The ultraviolet transmission of the pellicle film at 365 nm was measuredby using an ultraviolet-visible light spectrophotometer ("UV-260"supplied by Shimadzu Corp.) without any control light flux. The UVtransmission was 99.9%.

The pellicle film was exposed to monochromatic light (i-lineilluminance: 30.41 mW/cm²) of 365 nm from an ultra-high-pressure mercurylamp ("Multilight ML-501D/B" supplied by Ushio Electric Co.) over aperiod of 1,500 hours. When this exposed dose is converted into lightexposing times on the reticle surface of an i-line stepper, it equals tothe dose of 20,000,000 shots. The i-line dose is hereinafter expressedby the number of shots. After the UV exposure, the UV transmission at365 nm was measured in the same manner as described above. The UVtransmission was 99.9%. Thus, the UV transmission of the pellicle filmat 365 nm was not reduced by the exposure to the i-line.

EXAMPLE 2

A pellicle film was made from the reduced polyvinyl propional (RVP-2)prepared in Synthesis Example 3, by a procedure similar to that employedin Example 1. The UV transmission at 365 nm of the pellicle film,determined in the same manner as described in Example 1, was 99.9%.

The pellicle film was exposed to i-line of a mercury arc light at a doseof about 20,000,000 shots in the same manner as described in Example 1.The UV transmission at 365 nm of the pellicle film was not reduced bythe i-line exposure.

EXAMPLE 3

A pellicle film was made from the reduced polyvinyl propional (RVP-3)prepared in Synthesis Example 4, by a procedure similar to that employedin Example 1. The UV transmission at 365 nm of the pellicle film,determined in the same manner as described in Example 1, was 99.9%.

The pellicle film was exposed to i-line of a mercury arc light at a doseof about 20,000,000 shots in the same manner as described in Example 1.The UV transmission at 365 nm of the pellicle film was not reduced bythe i-line exposure.

EXAMPLE 4

A pellicle film was made from the reduced polyvinyl propional (RVP-4)prepared in Synthesis Example 5, by a procedure similar to that employedin Example 1. The UV transmission at 365 nm of the pellicle film,determined in the same manner as described in Example 1, was 99.9%.

The pellicle film was exposed to i-line of a mercury arc light at a doseof about 20,000,000 shots in the same manner as described in Example 1.The UV transmission at 365 nm of the pellicle film was not reduced bythe i-line exposure.

EXAMPLE 5

A pellicle film was made from the reduced polyvinyl butyral (RVB-1)prepared in Synthesis Example 7, by a procedure similar to that employedin Example 1. The UV transmission at 365 nm of the pellicle film,determined in the same manner as described in Example 1, was 99.9%.

The pellicle film was exposed to i-line of a mercury arc light at a doseof about 20,000,000 shots in the same manner as described in Example 1.The UV transmission at 365 nm of the pellicle film was not reduced bythe i-line exposure.

EXAMPLE 6

A pellicle film was made from the reduced polyvinyl propional (RVP-5)prepared in Synthesis Example 8, by a procedure similar to that employedin Example 1. The UV transmission at 365 nm of the pellicle film,determined in the same manner as described in Example 1, was 99.9%.

The pellicle film was exposed to i-line of a mercury arc light at a doseof about 20,000,000 shots in the same manner as described in Example 1.The UV transmission at 365 nm of the pellicle film was not reduced bythe i-line exposure.

EXAMPLE 7

A pellicle film was made from the reduced polyvinyl propional (RVP-6)prepared in Synthesis Example 9, by a procedure similar to that employedin Example 1. The UV transmission at 365 nm of the pellicle film,determined in the same manner as described in Example 1, was 99.9%.

The pellicle film was exposed to i-line of a mercury arc light at a doseof about 20,000,000 shots in the same manner as described in Example 1.The UV transmission at 365 nm of the pellicle film was not reduced bythe i-line exposure.

EXAMPLE 8

A pellicle film was made from the reduced polyvinyl propional (RVP-7)prepared in Synthesis Example 10, by a procedure similar to thatemployed in Example 1. The UV transmission at 365 nm of the pelliclefilm, determined in the same manner as described in Example 1, was99.9%.

The pellicle film was exposed to i-line of a mercury arc light at a doseof about 20,000,000 shots in the same manner as described in Example 1.The UV transmission at 365 nm of the pellicle film was not reduced bythe i-line exposure.

COMPARATIVE EXAMPLE 1

A pellicle film was made from the polyvinyl propional (PVP-1), which wasprepared in Synthesis Example 1 and was not subjected to a reductiontreatment, by a procedure similar to that employed in Example 1. The UVtransmission at 365 nm of the pellicle film was 99.9%.

The pellicle film was exposed to i-line of a mercury arc light in thesame manner as described in Example 1 and, when the dose reached about10,000,000 shots, the UV transmission at 365 nm of the pellicle film wasreduced to 99.0%, i.e., the pellicle became of no practical use.

COMPARATIVE EXAMPLE 2

A pellicle film was made from the polyvinyl butyral (PVB-1), which wasprepared in Synthesis Example 6 and was not subjected to a reductiontreatment, by a procedure similar to that employed in Example 1. The UVtransmission at 365 nm of the pellicle film was 99.9%.

The pellicle film was exposed to i-line of a mercury arc light in thesame manner as described in Example 1 and, when the dose reached about7,500,000 shots, the transmission at 365 nm of the pellicle film wasreduced to 99.0%, i.e., the pellicle became of no practical use.

What is claimed is:
 1. A film for a pellicle consisting essentially of aresin prepared by treating with a reducing agent a polyvinyl acetalresin represented by the following formula (1): ##STR4## wherein R is alinear or branched alkyl group having 1 to 3 carbon atoms, and x, y andz are numbers such that the content of the vinyl acetate units,expressed by y/(x+y+z), is not larger than 0.1 and the degree ofacetalization, expressed by z/(x+y+z), is at least 0.6.
 2. A film for apellicle according to claim 1, wherein the reduction of the polyvinylacetal is effected by using lithium aluminum hydride.
 3. A film for apellicle according to claim 1, wherein the reduction of the polyvinylacetal is effected by catalytic hydrogenation reduction using aRanie-nickel catalyst.
 4. A film for a pellicle according to claim 1,wherein the reduction of the polyvinyl acetal is effected by catalytichydrogenation reduction using a palladium carbon catalyst.
 5. A film fora pellicle according to claim 1, wherein the reduction of the polyvinylacetal is effected by using a solution of metallic sodium inhexamethylphosphoric triamide.